Control system for driers



O 1950 M. J. ERISMAN EI'AL 2,525,535

CONTROL SYSTEM FOR DRIERS Filed July 19. 1945 2 Sheets-Sheet 1 3mmMaw/291573070 1950 M. J. ERISMAN ETAL CONTROL SYSTEM FOR DRIERS Filed.July 19, 1945 2 Sheets-Sheet 2 Patented Oct. 10,

UNITED STATES PATENT OFFICE CONTROL SYSTEM FOR DRI ER Maurice J.Erisman, Dallas, Tex., and Wesley Coffin, flak Park, Ill., assignors toLink-Belt Company, a corporation of Illinois Ap ilication July 19, 1945,Serial No. 606,010

(Cl. a l -54) 9 Claims.

, j V 1 L This invention relates to new and useful improvements incontrol systems for dryers of the type which handle flowable solids inbulk and which pass the drying medium through a bed or such solids.

There is one type or class of dryer now in common commercial use whichprogressively advances a bed of flowable solids through a treatmentchamber, usually a rotary kiln or drum, and which effects forcedcirculation of the drying medium upwardly through the bed of solids. Thepatent to Gustav Bojner et al., No. 1,709,456, issued April 16, 1929, isillustrative of this type of dryer.

It, of course, is necessary, with this type of dryer, to control thetemperature of the drying medium that is circulated through thetreatment chamber in accordance with the amount of drying that isnecessary for each particular material being treated. It, also, isnecessary to control the amount or volume of drying medium that iscirculated through the material at the desired, constant temperature.

It is the primary object of this invention to provide a control systemfor dryers in which the amount of drying medium delivered to thetreatment chamber, for passage through the bed of material, isdetermined by, or regulated in accordance with, variations intheresistance offered by the bed of material to the flow of the dryingmedium therethrough and variations in the temperature of the mediumexhausted from the treatment chamber.

A more specific object of the invention is to provide a control systemfor dryers in which a drying medium flow regulating inlet damper ismoved through a predetermined throttling range by a suitable motor, theoperation of which is controlled by variations in the inlet air staticpressure and the exhaust air temperature.

A further object of the invention is the provision of a control systemfor dryers of the type in which the drying medium is circulated througha bed of material in a treatment chamber by means of two circulatingfans which are connected respectively to the inlet and the exhaust ductsof said chamber, said control system operating to automatically maintainthe desired drying eifect at all times by varying the amount of dryingmedium forcibly delivered to the treatment chamber in accordance withvariations in the resistance offered by the bed of material to the flowof the drying medium therethrough and the temperature of the mediumexhausted from the treatment chamber, and by hood for the treatmentchamber.

Other objects and advantages of the invention will be apparent duringthe course of the following description.

In the accompanying drawings forming a part of this specification and inwhich like numerals are employed to designate like parts throughout thesame:

Figure 1 is a side elevational view, partly broken away, of a dryer withthe form of control system embodying this invention operativelyassociated therewith,

Figure 2 is a plan view of the dryer and the control system illustratedin Fig. 1, and

Figure 3 is a diagrammatic view illustrating a portion of the dryer andall of the instrumentalities embodying said control system.

In the drawings, wherein for the purpose of illustration is shown thepreferred embodiment of this invention, the reference character Adesignates the drying chamber of a conventional form of dryer of thegeneral type illustrated by the above referred to patent to GustavBojner et al. This drying chamber consists of a rotatable kiln or drumwhich is suitably supported by the rings a and the rollers b in theconventional manner. A suitable drive, not shown, is provided forrotating the drying chamber A.

A feed chute P, or the like, introduces the material to be dried intothe central, material receiving compartment at the feed end of thedrying chamber A.

As is illustrated by the above identified patent, the interior of thedrying chamber A is provided I with an annular series of louvres Rattached to the inner surface of the kiln or drum shell. These louvres Rhave their free longitudinal edges overlapping the next adjacent louvresbut spaced therefrom. These louvres R provide an equal number oflongitudinally extending ducts through which the drying medium, which isusually heated air, passes for delivery to the central, materialreceiving compartment through the spaces left between the freelongitudinal edges of the louvres andthe next adjacent louvres R. Thisannular series of internal louvres functions as a support for a bed T ofthe material being dried. As the drying chamber A rotates, this bed T ofmaterial cascades circumferentially of the charm 3 her and the materialis progressively advanced from the feed end to the discharge end of thechamber.

The hot drying air is delivered to the feed end of the drying chamber Aby means of the inlet box B. The patent issued to Charles A. Schneider,Number 2,253,098, on August 19, 1941, illustrates in detail the type ofair inlet box that is conventionally provided for this type of dryer.This air inlet box functions to deliver the drying air to the adjacentends of the air ducts that are formed by the louvres R supporting thebed T of material being dried. It will be recognized that as the dryingchamber A rotates, the air inlet box B will be progressively placed incommunication with the angularly advancing air ducts that are formed bythe louvres R.- In this way, the air inlet box is connected at all timesto only the air delivery ducts that are formed by the louvres supportingthe bed T of material.

The airinlet duct C is illustrated as being connected to the inlet box Bat one end and to the outlet-of the circulating fan D at its oppositeend. It is the function of this fan to force the hot drying air throughthe duct C and the inlet box B in the drying chamber A.

Fig. 2 illustrates a heat exchange unit E through which steam iscirculated by means of the pipes e. This steam, by passing in heatexchange relation with the air that is forced through the inlet duct C,Will heat the air to the desired temperature. Although no such controlis illustrated, it will be understood that the temperature of the airthat has passed through the heat exchange unit E may be maintained at aV desired, constant temperature by regulating the flow of steam throughthe unit E. This automatic regulation of the flow of steam conventionallis obtained by a suitable instrument that operates in response to thetemperature of the air passing through the inlet box B.

Although the drawings illustrate the use of a steam heat exchange unitfor heating the inlet air, it will be understood that dryers of thistype also conventionally employ gas, coal, and oil fired furnaces, inplace of steam, for heating the inlet air.

Theproperly dried material, and the exhaust drying air are dischargedfrom the remaining end of the drying chamber A into the discharge orexhaust'hood F. The dried material passes out of the hood F through thedischarge spout 1 while the exhaust drying air leaves the hood F by wayof the exhaust duct 9. This exhaust drying air carries with it the dustor fines which are released from the material while it is being dried inthe chamber A.

This dust ladened, exhaust air passes from the exhaust duct g into thedust collector G where the dust is separated out for discharge throughthe lower end h of the collector G. The upper end of the collector isconnected b the exhaust duct 7c to'the inlet of the air circulating fanL. The outlet for this exhaust fan L is connected to the stack by meansof the duct m.

All of the above detail description refers to the elements of aconventional dryer installation for handling flowable solids. Thedescription that now will be presented refers to the novel controlsystem embodying this invention.

A suitable vane type damper is illustrated in Fig. 2 as being associatedwith the air inlet for the circulating fan D. It will be understood,however, that a louvre type damper may be used on the downstream side ofthe fan, is desired. B3

adjusting the position of this damper, the amount of air admitted to thefan can be regulated or varied. This damper, therefore, will function tothrottle the circulated drying air over a range that has as its limitsthe closed and opened positions of the damper. It willbe noted at thispoint that dampers of the vane type do not completely shut off or stopthe admission of air when they are in their so-called closed position.If a louvre type damper is used, it can be adjusted to only partiallyclose.

This damper 5 is moved through its operating range by means of suitablelink and lever mechanism 6 and the diiferential pressure motor I. Thismotor is diagrammatically illustrated in Fig. 3 as consisting of acylinder 8 with a reciprocating piston 9 mounted therein. This piston isloaded by the spring H) in the direction the piston must move to arrangethe damper 5 in its closed position. The opposite side of the piston 9is subjected to the action of compressed air that is delivered to themotor cylinder 8 by means of the pip line I l.

The supply of compressed air delivered to the damper operating motor 1is regulated or controlled by two instrumentalities which are connectedin series in the compressed air suppl line leading from a source ofsupply to the motor 1. Both of these instrumentalities operate on thebasic principle of venting or exhausting to the atmosphere variableamounts of the motor operating compressed air. These instrumentalities,therefore, are each capable of venting all of the damper motor operatingcompressed air; venting any desired fractional portion of suchcompressed air supply; or completely preventing the venting of any ofthe compressed air supply. Because the inlet air controlling damper 5 ismoved from its closed position toward its fully opened position by theaction Of the compressed air opposing the action of the spring l0, itwill be seen that each one of these control instruments can functionindependently of th other to cause the damper 5 to move toward itsclosed position, and that neither one of these control instrumentalitiescan bring about the movement of the damper 5 toward its fully openposition if the second control instrumentality is set or conditioned tovent the entire supply of compressed air.

These two control instrumentalities, as has been noted above, arearranged in series in the compressed air supply line for the damperoperating motor I and for that reason the action of each one of theseinstrumentalities, in controlling the delivery of compressed air to themotor I, is superimposed on the action of the other instrumentality.However, each one of these two instrumentalities obtains its actuatingforce, or motivating power, independently of the other instrumentality.Expressed in another way, one of these compressed air supply controllinginstrumentalities is actuated in response to variations in the inlet airstatic pressure prevailing in the inlet box B while the secondinstrumentality is actuated in response to variations in the temperatureof the exhaust drying air passing through the discharge or exhaust hoodF.

By referring to Fig. 3, it will be seen that the compressed air supplpipe line H extends from the damper operating motor I to the casing ofthe pressure venting or regulating valve l2. This valve is onlydiagrammatically illustrated because pressure venting or regulatingvalves of this character can be obtained from several differentmanufacturers. The valve casing 12 is chambered or hol-lowedout at H forcommunication with the end of thecompressed air suppl pipe line I l. Asuitable valve seat I4 is located in the valve casing l2 and; hasoperatively associated therewith the valve [-5. This valve and valveseat are intended to cooperatewith each other, by being moved relativeto each other, for varying the amount ofcompressed air that is vented tothe atmosphere from the interior or chamber l3 of the valve casing. Thatis to say, with the valve L5 in its fully opened position, all of thesupply of compressed air can be vented to the atmosphere while none ofthe supply of compressed air will'be vented to the atmosphere-when thevalve I5 is fully seated. In intermediate positions, this valve isintended tovent to the atmosphere different fractional portions. ofthecompressed air supply.

The valve l 5ais attached to the-stem lli which extends through anenlarged opening I 'l formed in the diaphragm casing IS. The end of thisvalve stem is connected to the diaphragm t9 that is clamped at itsoutermargin between the halves of the diaphragm; casing 18. A lever 20 ispivotally connected to the valvestem IS, with a lost motion connectionto allow for relative transverse movement, andis pivoted on a fixedsupport at 2 I. A spring 22 is operatively connected to the free endofthe lever 20- and functions to return the valve stem,and-its-associated valve l5, tothe normal positions for these elements,as shown in Fig; 3.

' The enlargedopeningll formed in one side of the diaphragm casinglstunctions to subject one face or side of the diaphragm I9 to atmosphericpressure. The remaining face or side of the diaphragm I9 is intended-tobe subjected to the inlet air static pressure prevailing in the inletbox B of the dryer chamber A. This-isaccomplished by the tubing 23 whichcommunicates at one end with the interior of thediaphragm casing is andat its other end with the interior of the air inlet box B.

This compressed air supply controlling instrumentality operates in thefollowing manner:

It was notedabove that the inlet air controlling damper Eris neverclcsedso completely as to preventiany airfrom being delivered to the airinlet box Bi Therefore, whenever the air circulating fan D is operating,someair will be supplied to the drying i chamber A and there will be alow static pressure within theair inlet box B. This static pressureprevailing in the inlet box B will be applied-to the sealed or closedhalf-of the diaphragm casing 13 and will act upon the associatedface orside-of the diaphragm is. When the static pressure within the sealedhalf of the diaphragm casing is exceeds the load imposed by the spring22, the diaphragm l9 will be flexed and the valve l5 will be movedtoward its associated seat l4. When the staticpressure Within theinlet'box B reaches a predetermined maximum, the diaphragm I9 will beflexed sufliciently to completely close the venting valve l2; orcompletely engagethe valve l5'with its seat I4; This inward'movement ofthe valve l5, from its illustrated position toward the valve seat a l 4;will function to reduce the amountof the compressed air supply that isvented bythis control instrumentalityi When the valve [5 is fullyengaged with its seat I 4, none of the compressed air supplyfor thedamper operating motor I will be vented to the atmosphere by thisinstrumentality, and the motor will function to open the damper 5 toincrease thesupply of air to the air inlet box 6 B. This increase in thesupply of air isin re sponse to the increase in resistance to the flowof air through the material in the receiving compartment; the increaseof resistance being indicated by the increase in the static pressurewithin the inlet box B. a i

The second instrumentality is of a type which is well known in the artand is generally referred to as an air-operated controller.Sevcraldifferent control instrument manufacturing companies produce thistype of controller and for that reason the diagrammaticdisclosurepresented by Fig. 3 of the drawings will be described only generally,and primarily for the purpose of explaining its broad principle ofoperation.

A compressed air supply line 24 extends from the airpressureregulatingvalve casing l2 to the valve casing 25 of this instrumentality. The

valve casing; 25 formsa part of a pilot valve-which also includes abellows-'26. connected by the valve rod 21* tothe valve head- 28 thatpartakes of straight line orrectilinear movement. This valve head 28 isadaptedto be moved in response to pressurevariations that are developedwithin the interiorof the bellows 26. These pressure variations willresult in movement of the valve head 28 between the illustratedposition, in which is closes thevent opening 29, and a position in whichit will close the opening 30 that communicates with the air supply pipe3| that extends to a suitable source ofsupply ofcompressed air, notshown. The source ofsupply ofcompressed air operates to provide asuitable compressed air pressure that usually falls within a range offrom 15 poundsto 25poundsl Branching off fromthe valve casing passage 32is a pipe line 33 havinga restriction 341ocated therein. Thisrestriction 34 functions to limit the flow of air from the supply pipe3! into the branch line 35 that communicates with the bellows 25 at oneend and is formed with a nozzle 36- at its other end. This restriction34- operates to pipe 3| that extends to the interior ofthe pilot valvecasing 25. This differential or ratio of air pressure is desirablebecause i the flow of air to the pipe line 35 is used for controlpurpose-s while the flow of air through the pipe line 24 is employed foroperating the motor 1 that actuates the control damper 5. If therestriction 34 were not used, the supply of control air to the nozzle36, for practical purposes, would be considered as unlimited and itwould be impossible to cause the pressure within the branch line 35 tobe dropped to atmospheric, whichis essential and isobtained bycontrolling the rate of discharge-of the air through the-nozzle 36.

A flapper 31 is employed forregulati'ng the amount ofcontrol air that isdischarged through the nozzle 36 as a small jet. To accomplishthisfunction, the flapper 31 is pivotally supportedat 38 and is adapted tobemoved about 0.002 of an inch relative to the discharge face of thenozzle 36 to effect the desired change in nozzle pressure from theminimum to the maximum.

The flapper 31 is loaded by the spring 39 to cause the flapper to moveaway from the nozzle 3B.-

This air-operated controller instrumentality is conventionally providedwith either an indicating pointer or a recording pen that isdiagrammatically represented at 40; This pointer or pen 4!) isillustrated as being actuated by a suit-- able temperature responsivecoil element ll that This bellows unit 26 is is connected by the tubing42 to the temperature bulb 43 which is located in the exhaust ordischarge hood F. Of course, the pointer or pen 4|] just as readily canbe actuated by a thermo-couple type of instrumentation, if desired.Therefore, the pointer or pen will be operated in response to variationsin the temperature of the exhaust drying air passing through the hood F.A suitable link and lever mechanism is diagrammatically illustrated at Mfor operatively connecting the pointer or pen 40 to the flapper 31. Itwill be appreciated, therefore, that indicating or recording movementsof the pointer or pen 40 will bring about corresponding, but greatlyreduced, movements of the flapper 31 relative to the face of the nozzle36.

The operation of this air-operated controller instrument will bepresented as follows:

A supply of compressed air at full operating pressure will be deliveredto the pilot valve casing by the supply pipe 3!. A substantially reducedsupply of air will be delivered to the nozzle 36 and to the interior ofthe bellows 26 by the pipe lines 33 and 35.

The flapper 31 cooperates with the nozzle 38 to vary the back pressurethat prevails within the bellows 26 by being moved into contact with ordifferent distances from the discharge face of the nozzle.

It has been pointed out above that the air pressure that is applied tothe piston 9 of the motor 1 determines the position of the inlet aircontrol damper 5. This air pressure that is applied to the motor I iscontrolled in part by the instrumentality that has just been described.This instrumentality operates to vary the air pressure applied to thedamper operating motor I by depending on a balance between the supply ofair to this control instrumentality by the supply pipe Si and theleakage of the air out of this instrumentality by way of the nozzle 36.If the supply of air to the instrumentality is greater than the leakagethrough the nozzle 36, the pressure applied to the bellows 26 willapproach the air supply pressure. If the leakage through the nozzle 36is greater than the air supply, the pressure within the bellows casing29 will approach atmospheric.

If we now consider that the flapper 3'! is positioned relative to thenozzle 38 in accordance with movements of the pointer or pen 40 and thatfor every position of this pointer or pen 4!] there will be provided adefinite control pressure within the bellows 26, it readily will be seenthat the valve head 28 will be caused to assume a given positionrelative to the supply pipe opening and the venting opening 29 for veryposition of the pointer or needle 40. Consequently, movements of thepointer or needle to the right or left relative toits control index willcause the pilot valve 25 to effect increases or decreases respectivelyin the air pressure supplied to the damper operating motor 1, subject tosuperimposed control effected by the static pressure regulatorinstrumentality previously described in detail.

The superimposed operations of the two control instrumentalities toregulate the amount of hot drying air admitted to the drying chamber Ain response to variations in inlet air static pressure and exhaust airtemperature will be described as follows:

Let it be assumed that the dryer installation has been shut down at theend of a working period and is started up after several hours ofidleness. an opportunity to cool off during this shut down period. If abed T of material was left in the drying chamber at the end of thepreceding working period, the material of this bed is more than likelycompletely dried throughout the entire length of the drying chamber as aresult of hav-' ing absorbed heat from the walls of the chamber, theinternal, annular series of louvres R, etc. This bed T of material, morethan likely, is excessively dried at the discharge end portion of thechamber.

When the dryer installation is first started up,

an excessive amount of the heatwill be extracted r from the drying airin raising the temperature of the drying chamber walls, etc., to thenormal working temperature. Consequently, the temperature of the exhaustair will be abnormally low and the position of the flapper 31 relativeto the nozzle 36 will be such as to provide a con-,

trol pressure in the bellows 26 whereby the valve head 28 will assume aposition relative to the venting opening 29 which will cause theair-operated controller to call for the delivery of an abnormally highamount of the drying air.

If this air-operated controller were the only instrumentality that wasoperating to regulate the air inlet control damper 5, an unusually largeamount of drying air would be delivered to the drying chamber A. Thisdelivery of an excessive amount of dryin air to the chamber A will causethe bed T of material to blow. The excessively dried condition of thebed T of material left in the chamber during the'shut down period willincrease the possibility of the bed blowing as a result of this deliveryof an excessive amount of the hot drying air.

With the static pressure regulator instrumentality operating in serieswith the air-operated controller, this blowing oi the material bed Twill be prevented. That is to say, the excessively dry condition of thestarting bed T of material will oiier less resistance to the flow of thedrying medium therethrough and for that reason a lower inlet air staticpressure willprevail in the inlet box B than will exist when the dryerinstallation is operating normally. Addi tionally, if the bed T ofmaterial in the drying chamber A should start to blow, the inlet airstatic pressure in the inlet box B will drop accordingly. This low inletair static pressure in the inlet box B will cause the static pressureregulator instrumentality to effect a, control of the inlet airregulator damper 5 which will prevent this damper from being opened inresponse to the control eiiected by the air-operated controller.

It will be apparent, therefore, that the two control instrumentalitieswill operate in combination to prevent the inlet air regulating damper 5from being opened sufliciently to permit too much air to be admitted tothe chamber A until both the static inlet pressure and the exhausttemperature stand at proper values. This dual regulation or control ofthe damper 5 will continue until the pressure and temperature conditionsat the opposite ends of the drying chamber are such that a state ofbalance will prevailwithin the drying chamber.

During the continued operation of the dryer, variations in the rate offeed of the material will cause the bed T of material in the chamber Ato increase or decrease in thickness and in the amount of resistance thebed offers to the flow of the drying air therethrough. Additionally,

The. drying chamber A will have had? if the moisture content of thematerial fed to the drying chamber varies, the amount of heat extractedfrom the drying air, during its passage through the bed of material,will vary. These variations in the condition of the bed T of materialwill cause the inlet air static pressure in the box B to changecorrespondingly and will cause the exhaust air temperature to change.The two control instrumentalities will effectively take care of thesevariations and the most efficient or efiective drying condition will beprovided in the chamber A.

A very close control over the amount of air admitted to the chamber isnecessary when the These types of materials will It has been pointed outabove that the drying air is forcibly exhausted from the drying chamberA by the air fan L. This exhaust fan should withdraw the exhaust dryingair at a rate which will maintain a slight negative pressure, or vacuum,in the discharge or exhaust hood F. If the exhaust fan L is not operatedin unison with the operation of the inlet fan D, unsatisfactory pressureconditions will prevail in the drying chamber A over the bed T ofmaterial. If the pressure over the bed T of material is higher thannormal, excessive dusting of the material will occur. If too low avacuum prevails in the chamber space, over the bed T of material, anexcessive loss of fine or light materials will occur through the exhauststack. Additionally, exhausting of the drying air at an improper ratewill create false temperature conditions in the :9

discharge or exhaust hood F.

To control the rate at which the drying air is exhausted by the fan L, acontrol damper 52 is provided in the outlet for the exhaust fan L. Thisdamper is automatically controlled by the differential pressure motor 53through the medium of a suitable train of mechanical ele ments 54 thatconnects the damper with the'rod 55 of the motor piston 56. This pistonis loaded in one direction by the spring 51 to effect opening of thedamper 52. The motor piston 56 is moved in the opposite direction, toeffect closing of the damper 52, by compressed air that is delivered tothe motor cylinder through the pipe line 58.

This pipe line leads to the valve casing 59 of a static vacuumregulator, This valve casing is chambered or hollowed out at 60 forcommunication with the pipe line 58. A second pipe line 6|- leads from asource of compressed air to the regulator valve 59.

The regulator valve 59, further, includes a valve head 62 thatcooperates with a valve seat 63 for controlling and varying the amountof the compressed air supply that is vented to the atmosphere from theregulator valve casing chamber 60. It will be appreciated that theposition of the damper operating motor piston 56 can be controlled bythe proper venting of the compressed air to the atmosphere.

The valve head *62 is moved into difierent positions with respect to itsseat 63 by the valve rod 64 that is connected to the lever 65 and to thediaphragm 65. This diaphragm is clam ed at its margin between the twohalves of the diaphragm casing 61. One side of the diaphragm issubjected to atmospheric pressure because of the large opening 68 thatallows for the passage of the valve stem. into this casing 61. Theremaining sideof the diaphragm 66 is subjected I I1 static ressure insaid air inlet box: for actuatin V to the static negative pressuraorvacuum, that prevail in the discharge or exhaust hood F through themedium of the tube 69. The opposite ends of this tube communicate withthe interior of the diaphragm casing 6'! and the interior of the hood F.The lever 65 is pivotally mounted at 15 at one end and is loaded at itsopposite end by the spring II.

It is to be understood that the form of this invention herewith shownand described isto be taken as a "preferred example of the same, and

that various changes in the shape, size, and arrangement of parts may berestorted to without departing from the spirit of the invention or thescope of the subjoined claims.

Having thus described the invention, we claim: 1, In a control systemfor dryers, the combination with a chamber through which :a bed ofmaterial to be driedrmayrbe advanced, a discharge hood communicatingwith one end of the drying chamber, and means for forcing hot drying airthrough the bed of material in the chamber including an air inlet boxcommunicating with the other end of said chamber, of an adjustabledamper for varying the amount of, hot air delivered by the aforesaidmeans, a motor for adjusting the damper toward either its fully openedor itsfully closed position, and means for conducting and regulating theflow of a motivating pressure fluid to said motor, said means comprisingfiow piping for said pressure fluid, two devices connected in series insaid flow piping for regulating the flow of the pressure fluid, eitherof said devices being operable independently, of the other to regulatethe flow of pressure fluid to-effect operation of the motor to move thedamper toward its fully closed position, both of said devices beingoperable jointly but not independently to regulate the flow of pressurefluid to effect operation of the motor to move the damper toward itsfully open position, fluid pressure operated means connected to the airinlet box and operable in response to variations in the air staticpressure in said box to actuate one of said devices, and temperatureoperated means connected to the discharge hood and operable in responseto variations in the temperature of the exhaust air passing through saidhood to actuate the other of said devices.

2. In a control system for dryers, the combination with a chamberthrough which a bed of material to be dried may be advanced, a dischargehood communicating with one end of the drying chamber, and means forforcing hot drying air through the bed of material in the cham berincluding an air inlet box communicating with the other end of saidchamber, of anadjustable damper for varying the amount of hot airdelivered by the aforesaid means, a motor including a piston loaded by aspring for movement in one direction for closing the damper and operatedby pressure fluid for movement in the other direction for opening thedamper, a pipe line for conducting pressure fluid to the motor, two flowcontrolling valves connected in series in said pipe line so as tooperate independently of each other to reduce the application ofpressure fluid to the motor to bring about closing movement of thedamper and so as to operate dependently of each other to increase theapplication of pressure fluid to the motor to effect opening movement ofthe damper, fluid pressure operated means connected to the air inlet boxand operating in response to variations in the air one of said valves,and temperature operated means connected to the discharge hood andoperable in response to variations in the temperature of the exhaust airpassing through the discharge hood for actuating the other of saidvalves.

3. In a control system for dryers, the combination with a chamberthrough which a bed of material to be dried may be advanced, a dischargehood communicating with one end of the drying chamber, and means forforcing hot dry-- ing air through the bed of material in the chamberincluding an air inlet box communicating with the other end of saidchamber, of a damper adjustable in opposite directions for increasingand decreasing the amount of hot drying air delivered by the aforesaidmeans, spring means for moving the damper in one of said directions,fluid pressure operated means for moving the damper in the other of saiddirections, a pipe line for supplying constant pressure to said fluidpressure operated means, two instrumentalities operating in series insaid pipe line to control the flow of fluid through the latter, meansresponsive to pressure and operatively connected to the air inlet boxfor effecting actuation of one of said instrumentalities in response tovariations in the air static pressure in said air inlet box, and meansresponsive to temperature and operatively connected to the dischargehood for effecting actuation of the other of said instrumentalities inresponse to variations in the temperature of the exhaust air flowingthrough the discharge hood.

4. In a control system for dryers, the combination with a chamberthrough which a bed of material to be dried may be advanced, a dischargehood communicating with one end of the drying chamber, an exhaust fanconnected to the discharge hood for withdrawing air therefrom, and meansfor forcing hot drying air through the bed of material in the chamberincluding an aireinlet box communicating with the other end of saidchamber, of an adjustable damper for varying the amount of hot airdelivered by the aforesaid means, a motor for adjusti g the dampertoward either its fully opened or its fully 'closed position, means forconducting and regulating the flow of a motivating pressure fluid tosaidmotor, said means comprising flow piping for said pressure fluid,two devices connected in series in said flow pining for regulating theflow of the pressure fluid, either of said devices being operableindependently of the other to re ulate the flow of pressure fluid toeffect operation of the motorto move the damper toward its fully closedposition, both of said devices being operable jointly but notindependently to regulate the flow of pressure fluid to efiect operationof the motor to move the damper toward its fully open position, fluidpressure operated means connected to the air inlet box and operable inresponse to variations in the air static pressure in said box to actuateone of said devices, and temperature operated means connected to thedischarge hood and operable in response to variations in the temperatureof the exhaust air passing through said hood to actuate the other ofsaid devices, and means responsive to pressure and operatively connectedto the discharge hood operating in response to variations in the staticpressure in the discharge hood for increasing or decreasing the rate ofwithdrawal of air from the discharge hood so as to maintain a slightnegative pressure in said hood.

5. In a control system for dryers, the comb nation with a chamberthrough which a bed of material to be dried may be advanced, a dischargehood communicating with one end of the drying chamber, an exhaust fanconnected to the discharge hood for withdrawing air therefrom, and meansfor forcing hot drying air through the bed of material in the chamberincluding an air inlet box communicating with the other end of saidchamber, of an adjustable damper for varying the amount of hot airdelivered by the aforesaid means, spring means for moving the dampertoward its fully closed position, fluid pressure operated means formoving the damper toward its fully opened position, a pipe line forsupplying constant pressure fluid to said fluid pressure 0D- eratedmeans, two instrumentalities operating in series in said pipe line tocontrol the flow of fluid through the latter, means responsive topressure and operatively connected to the air inlet box for effectingactuation of one of said instrumentalities in response to variations inthe air static pressure in said air inlet box, means responsive totemperature and operatively connected to the discharge hood foreffecting actuation of the other of said instrumentalities in responseto the variations in the temperature of the exhaust air flowing throughthe discharge hood, and means responsive to pressure and operativelyconnected to the discharge hood operating in response to variations inthe static pressure in the discharge hood for increasing or decreasingthe rate of withdrawal of air from the discharge hood so as to maintaina slight negative pressure in said hood.

6. In a control system for dryers, the combination with a chamberthrough which a bed of material to be dried may be advanced, a dischargehood communicating with one end of the drying chamber, and means forforcing hot drying air through the bed of material in the chamberincluding an air inlet box communicating with the other end of saidchamber, of an adjustable damper for varying the amount of hot airdelivered by the aforesaid means, spring means i for moving the dampertoward its fully closed position, air pressure operated means for movingthe damper toward its fully opened position, a pipe line for supplyingconstant pressure air to said air pressure operated means, twoinstrumentalities operating in series in said pipe line to control theflow of air through the latter, each of said instrumentalities includinga valve device for venting to the atmosphere variable amounts of saidpressure air, means responsive to pressure and operatively connected tothe air inlet box for effecting actuation of one of saidinstrumentalities in response to variations in the air static pressurein said air inlet box, and means responsive to temperature andoperatively connected to the discharge hood for effecting actuation ofthe other of said instrumentalities in response to variations in thetemperature of the exhaust air flowing through the discharge hood. I

'7. In a control system for dryers, the combination with a chamberthrough which a bed of material to be dried may be advanced, a dischargehood communicating with one end of the drying chamber, an exhaust fanconnected to the discharge hood for withdrawing air therefrom, and meansfor forcing hot drying air through the bed of material in the chamberincluding an air inlet box communicating with the other end of saidchamber, of an adjustable damper for varying the amou t 9? hot airdelivered by the aforesaid means, spring means for moving the dampertoward its fully closed position, air pressure operated means for movingthe damper toward its fully opened position, a pipe line for supplyingconstant pressure air to said air pressure operated means, twoinstrumentalities operating in series in said pipe line to control theflow of pressure air through the latter, each of said instrumentalitiesincluding a valve device for venting to the atmosphere variable amountsof said pressure air, meansresponsive to pressure and operativelyconnected to the air inlet box for effecting actuation of one of saidinstrumentalities in response to variations in the air static pressurein said air inlet box, means responsive to temperature and operativelyconnected to the discharge hood for effecting actuation of the other ofsaid instrumentalities in response to variations in the temperature ofthe exhaust air flowing through the discharge hood, and means responsiveto pressure and operatively connected to the discharge hood operating inresponse to variations in the static pressure in the discharge hood forincreasing or decreasing the rate of withdrawal of air from thedischarge hood so as to maintain a slight negative pressure in saidhood.

8. A control system for dryers, comprising a chamber through which a bedof material to be dried may be advanced, a discharge hood communicatingwith one end of the drying chamber, means for forcing hot drying airthrough the bed of material in the chamber including an air inlet boxcommunicating with the other end of said chamber, a motor operateddamper associated with said first mentioned means for varying the amountof drying air delivered by the air forcing means, means for conducting amotivating medium to the damper motor, a first control instrumentalityconnected in said conducting means and provided with a tube terminatingin said inlet box for applying the air static pressure in the inlet boxto said first control instrumentality to regulate the flow of themotivating medium to the damper motor, and a second controlinstrumentality connected in said conducting means on the input side ofsaid first control instrumentality and provided with a temperatureresponsive element located in heat exchange relation to said dischargehood to regulate the fiow of the motivating medium to said first controlinstrumentality, said control instru mentalities being operable jointlybut not independently to regulate the flow of motivating medium to andeffect operation of the damper motor to cause more air to be deliveredby said air forcing means, said damper motor including means forconstantly urging the motor to move the damper in a direction oppositeto that in which the damper is moved by the motor when actuated by saidmotivating medium.

9. A control system for dryers, comprising a chamber through which a bedof material to be dried may be advanced, a discharge hood communicatingwith one end of the drying chamber, an exhaust fan connected by anexhaust duct to the discharge hood for forcibly withdrawing airtherefrom, means for forcing hot drying air through the bed of materialin the chamber including an air inlet box communicating with the otherend of said chamber, a first motor operated damper associated with saidfirst mentioned means for varying the amount of drying air delivered bythe air forcing means, means for conducting a motivating medium to themotor of said first damper, a first control instrumentality connected insaid conducting means and provided with a tube terminating in said inletbox for applying the air static pressure in the inlet box to said firstcontrol instrumentality to regulate the flow of the motivating medium tothe motor of said first damper, a second control instrumentalityconnected in said conducting means on the input side of said firstcontrol instrumentality and provided with a'temperature responsiveelement' located in heat exchange relation to said discharge hood toregulate the flow of the motivating medium to said first controlinstrumentality, said control instrumentalities being operable jointlybut not independently to regulate the flow of the motivating medium tosaid first control.instrumentality, said control instrumentalities beingoperable jointly but not independently to regulate the flow ofmotivating medium to and efiect operation of the motor of said firstdamper to cause more air to be delivered by said air forcing means, saidfirst damper motor including means for constantly urging the motor tomove the damper in a direction opposite to that in which the damper ismoved by the motor when actuated by said motivating medium, a secondmotor operated damper associated with said exhaust duct for varying theamount of air withdrawn by said exhaust fan, means for conducting amotivating medium to the motor of said second damper, and a thirdcontrol instrumentality connected in said last mentioned conductingmeans and provided with a tube terminating in said discharge hood forapplying the pressure in said discharge hood to said third controlinstrumentality to regulate the flow of said motivating medium to themotor of said second damper to increase or decrease the rate ofwithdrawalof air so as to maintain a slight negative pressure in saidhood.

MAURICE J. ERISMAN. WESLEY W. COFFIN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 952,775 Whitlatch Mar. 22, 19101,750,813 Oakley et a1 Mar. 18, 1930 1,985,050 Merle Dec. 18, 19342,109,512 Stacey Mar. 1, 1938 2,150,367 Smith Mar. 28, 1939 2,160,956Cheesman June 6, 1939 OTHER REFERENCES The Link Belt Roto-Louvre Dryer,Book No. 1911, copyright 1941 by Link Belt C0.. pages 18 and 19.

